Distillation processes are very widely performed in many fields including the petrochemical industry. Multi-stage distillation columns are used in order to perform fractional distillation more precisely. When a usual distillation process is performed using multi-stage distillation columns, there may arise the problem that energy consumption is very large. Accordingly, internal heat exchange-type distillation columns have been proposed in order to utilize the energy efficiently. For example, Japanese Laid-Open Patent Publication No. 8-131704 discloses an internal heat exchange-type distillation column having a structure in which a single tube is inserted in the central section of a cylindrical main body, which forms a concentric double-tube structured column having a rectifying section inside the single tube and a stripping section outside thereof. With this concentric double-tube structured internal heat exchange-type distillation column, the single tube, i.e. the inner tube is divided into a plurality of blocks and forms a multi-stage structure in which a plurality of blocks are stacked in the vertical direction. The rectifying section of one block is linked to the rectifying section of an adjacent block by a connecting tube, and the heat of the rectifying section is transferred to the stripping section via the wall of the single tube and the wall of the connecting tube.
WO 2004/002602 describes an internal heat exchange-type distillation column with good efficiency of heat transfer, whose trays can be washed easily and for which maintenance involving periodic inspections can be easily performed. The distillation column design is such that the stripping section and the rectifying section can be opened and closed, and the distillation column is provided with heat transfer means projecting into the stripping section from the surface of the rectifying section that is in contact with the stripping section, and the heat of the rectifying section is transferred through the heat transfer means. The efficiency of heat transfer of this distillation device is higher than that of the device disclosed in Japanese Laid-Open Patent Publication No. 8-131704. However, the efficiency of heat transfer still remains to be improved.